The wet chemical etching of silicate glasses with aqueous hydrofluoric acid-containing media, for example to polish the surfaces of the glass or to strengthen the glasses via the removal of surface flaws, is well known. HF-containing media are capable of dissolving silica and other constituents of silicate glasses, although precipitates of those constituents, commonly referred to as “sludge”, are generally produced as by-products of that dissolution. Depending upon the compositions of the media and the glasses being treated, compounds such as sulfates, bisulfates, fluorides and silicofluorides of the cationic constituents of the glasses can be precipitated.
These precipitates can be deposited on the surfaces of glasses being treated to form firmly adherent layers that can progressively reduce the etching rate. In addition, precipitate buildup on processing tools such as squeegee rollers and acid bath cooling coils can cause equipment malfunctions and/or damage to the glass surfaces. The resulting need to clean tooling and/or the surfaces of glass articles being treated eventually requires that the etching processes be interrupted or terminated, with substantial decreases in production rates and increases in labor costs.
A number of measures to address the problem of sludge buildup in glass etching media have been proposed. In one approach, concentrations of sodium and/or potassium ions in the etching bath resulting from the dissolution of glass constituents are continuously reduced. The desired reductions may be achieved through one or a combination of measures including: (i) adding fluorosilicic acid to the bath; (ii) precipitating silicofluorides of the alkali ions and filtering off the precipitates; (iii) mercury cell electrolysis of the bath to form sodium and/or potassium amalgams, (iv) bath electrolysis in a cell having a cation semi-permeable membrane; and (v) utilizing alkali metal ion exchangers. An alternative ion-exchange approach involves treating the etching solution with a weak anion exchange resin of the polyamine type to remove SiF62− ions, thereby reducing the sludge-forming capacity of the solution.
Mechanical approaches for controlling sludge buildup include the use of a supplemental buffer tank for carrying out a filtration of precipitates from etching solution circulated from the main etching tank. In a variation on the approach of maintaining an effective HF concentration in the bath, fluoride gas generated during the etching process is collected and condensed to produce supplemental HF for recycling back into the etchant tank. Alternatively, it has been proposed to neutralize at least a portion of the hydrofluosilicic acid generated during etching while concurrently adding sufficient hydrofluoric acid to the bath to maintain the initial concentration of free hydrofluoric acid therein. Hydrofluorosilicic acid neutralization is achieved through the addition to the bath of a soluble potassium salt such as potassium carbonate, precipitating an insoluble potassium silicofluoride within the bath which can then be efficiently removed by filtration.
As is apparent from the foregoing, however, most of the proposed solutions to the problem of sludge buildup involve the design and construction of supplemental systems that are often quite costly and that may not be practical where operating spaces are limited. In addition, new chemicals can be required that are expensive, or that have the potential for changing bath chemistry in ways that may adversely affect etching efficiencies or lead to the formation of new, possibly undesirable, by-products. Thus, there remains a need for improved methods for addressing the problem of sludge in an efficient and economical manner.